Electrical submersible pumps are used to convey large volumes of fluid from wells typically for hydrocarbon production. Normally an electrical submersible pump assembly will comprise a rotary pump and a downhole electrical motor. The rotary pump may be a centrifugal pump made up of a large number of centrifugal stages of impellers and diffusers. Alternately, a progressing cavity pump may be utilized in some circumstances.
The motor is filled with a dielectric motor oil or lubricant. The motor oil expands when the temperature rises, which normally occurs when lowering a pump into a well. Also, heat of the motor during operation causes the temperature to rise. The expansion of the oil could exceed the volume capacity of the motor, causing a leak. To avoid this occurrence, a seal section is connected between the motor and the pump. The seal section has an inlet for admitting well fluid and a flexible barrier to separate the well fluid from the lubricant and equalize the pressure between the lubricant contained in the motor and the well bore fluid. The seal section has a vent that allows the motor to vent excess oil into the well environment if the volume of oil increases beyond the volume capacity of the assembly.
Also, commonly the seal section will have a thrust bearing to take thrust load from the pump above. Conventional seal sections thus have four basic functions: a) equalizing pressure between the well bore and inside the motor; b) providing a reservoir for the motor oil; c) compensating for the expansion of oil due to temperature increase; and d) taking the thrust load from the pump above.
One problem with existing seal sections is that if a leak occurs, well fluid will enter the motor and cause an electrical short, thus destroying the equipment. To avoid this occurrence, in some cases, several seal sections are coupled together, with each operating independently of the other. In that arrangement, if the top seal section should leak and fail, the underlying seal sections will continue to protect the motor. Redundant seal sections are costly, however, and only add an additional amount of time before failure eventually occurs. Often, if the top seal section fails, vibration and leakage will also cause failure out of the other seal sections in fairly short order.